Video Card recommendations for about €150/$181.41/£103.55?

Like the title says I'm looking for a video card for about €150. My motherboard is model ORD203, from Dell and the chipset is 945 express(thanks for clearing that up DonNaguel) I have a Radeon x300 at the moment but I think it's integrated, it came with the pc.
All my other specs are in my profile.
I suppose have a few questions: Can anyone tell me if there's a way to see if I have a free slot or do you need me to open this thing up?
Is there a good site to buy stuff like this near me(e.g. England)?
I want to play Half Life 2 more comfortably, that's the goal at the moment. Is that possible for €150?

I have already tried town, I'm just back, and no luck. I tried three shops: the first, Maplins(RBS would know this place) was selling a 6600 512mb as their best card, for €169
The second one, Peats, was closed.
The third one, Dixons, had one graphics card, and they said they shouldn't be selling them any more, but they had this one left. It was another 6600, for €229 puke:

So, in conclusion, I'm really looking for help on this one. I don't care if you know nothing, please post anyway, my last thread bombed and I want to get this as soon as possible, like I said, if I had found one in town I would've gotten it today. Thanks.